Re: [libvirt] Is it a workqueue related issue in 2.6.37 (Was: Re: blkio cgroup [solved])

On Fri, Feb 25, 2011 at 04:03:29PM +0100, Tejun Heo wrote: > Hello, > > On Fri, Feb 25, 2011 at 09:57:08AM -0500, Vivek Goyal wrote: > > blk_throtl_work() calls generic_make_request() to dispatch some bios and I > > guess blk_throtl_work() has been put to sleep because threre are no request > > descriptors available and CFQ is frozen so no requests descriptors get freed > > hence blk_throtl_work() never finishes. > > > > Following caught my eye. > > > > ksoftirqd/0-3 [000] 1640.983585: 8,16 m N cfq4810 slice > > expired t=0 > > ksoftirqd/0-3 [000] 1640.983588: 8,16 m N cfq4810 > > sl_used=2 disp=6 charge=2 iops=0 sect=2080 > > ksoftirqd/0-3 [000] 1640.983589: 8,16 m N cfq4810 > > del_from_rr > > ksoftirqd/0-3 [000] 1640.983591: 8,16 m N cfq schedule > > dispatch > > sshd-3125 [004] 1640.983597: workqueue_queue_work: work > > struct=ffff88102c3a3110 function=flush_to_ldisc workqueue=ffff88182c834a00 > > req_cpu=4 cpu=4 > > sshd-3125 [004] 1640.983598: workqueue_activate_work: work > > struct ffff88102c3a3110 > > > > CFQ tries to schedule a work and but there is no associated > > "workqueue_queue_work" trace. So it looks like that work never got queued. > > > > CFQ calls following. > > > > cfq_log(cfqd, "schedule dispatch"); > > kblockd_schedule_work(cfqd->queue, &cfqd->unplug_work); > > > > We do see "schedule dispatch" message and kblockd_schedule_work() calls > > queue_work(). So what happended here? This is strange. I will put one > > more trace after kblockd_schedule_work() to trace that function returned. > > It could be that the unplug work was already queued and in pending > state. The second queueing request will be ignored then. So, I think > the problem is that blk_throtl_work() occupies kblockd but requires > another work item (unplug_work) to make forward progress. In such > cases, forward progress cannot be guaranteed. Either > blk_throtl_work() or cfq unplug work should use a separate workqueue. Ok, that would make sense. So blk_throtl_work() can not finish as CFQ is not making progress and no request descriptors are being freed and unplug_work() is not being called because blk_throtl_work() has not finished. So that's cyclic dependency and I should use a separate work queue for queueing throttle related work. I will write a patch.